1. Field of the Invention
The present invention relates to a resonator apparatus, in particular to a resonator apparatus having a piezoelectric resonator and an associated acoustic reflector.
2. Description of the Related Art
In FIG. 1 a resonator apparatus comprising a piezoelectric resonator 10 is shown by way of example. The piezoelectric resonator 10 comprises a piezoelectric layer 12 as well as a first electrode 14 and a second electrode 16. An acoustic Bragg reflector 18 comprising a plurality of layers 181 to 187 is arranged adjacent to the piezoelectric resonator 10. Further, a substrate 20 is provided. The reflector 18 is arranged between the substrate 20 and the piezoelectric resonator 10. The layers 181, 183, 185 and 187 of the acoustic reflector are layers having a low acoustic impedance, and the layers 182, 184, and 186 are layers having a high acoustic impedance.
In piezoelectric thin film resonators (TFBAR=thin film bulk acoustic wave resonator, or FBAR=film bulk acoustic wave resonator, BAW=bulk acoustic wave) the sound wave that has been excited in the piezoelectric layer (piezo layer—see reference numeral 12 in FIG. 1) has to be insulated acoustically from the substrate 20—over which the device is constructed—to guarantee functionality.
In the prior art two methods are known for the insulation. The first method is to remove the substrate 20 or an appropriate sacrificial layer, respectively, beneath the device. In this case the resonator forms a thin self-supporting structure (membrane or bridge). The disadvantage of this procedure is that the resulting structure is very sensitive and difficult to process further, in particular with regard to the packaging of such a structure.
The second method of acoustically insulating the device from the substrate 20 is shown in FIG. 1, in which the device is constructed on the acoustic Bragg reflector 18. As has been mentioned, the latter comprises a sequence of layers 181 to 187 having alternating high and low acoustic impedances. With regard to the structure shown in FIG. 1, reference is made to the U.S. Pat. No. 4,166,967 as well as to the article by K. Lakin in Appl. Phys. Lett. 38, 1981, pp. 125-127. Further, reference is made to the article by G. D. Mansfeld and S. G. Alekseev, in Ultrasonics Symp. Proc., Vol. 2, 1997, pp. 891-894.
In piezoelectric resonator apparatuses as have been described with the help of FIG. 1, for example, the optimum layer thickness of the individual layers for a given operating frequency f0 is around a quarter of the acoustic wavelength λac in the material or in the layer, respectively, and in accordance with the following condition (1) the optimum layer thickness results to be:                               d          opt                =                                            λ                              a                ⁢                                                                   ⁢                c                                      4                    =                                                    v                                  a                  ⁢                                                                           ⁢                  c                                                            4                ·                                  f                  0                                                      =                                          Z                                  a                  ⁢                                                                           ⁢                  c                                                            ρ                ·                4                ·                                  f                  0                                                                                        (        1        )            with:                vac=speed of sound in the layer being looked at,        Zac=acoustic impedance of the layer being looked at, and        ρ=the density of the material of the layer being looked at.        
The advantage of using the acoustic Bragg reflector is that the resonators manufactured by using this reflector have a high mechanical stability. The present invention also relates to this type of acoustic decoupling.
Typical materials with high acoustic impedance are metals such as tungsten (W), platinum (Pt), molybdenum (Mo) or gold (Au). Materials with low acoustic impedance are, for example, silicon dioxide (SiO2) or aluminum (Al).
When realizing the Bragg reflectors 18 shown in FIG. 1 the problems stated below may arise in several aspects.
First, the realization of the above-mentioned layer thicknesses dopt may be problematic for technological reasons. One example for this is the limitation of the realizable layer thicknesses due to layer stresses that are produced during deposition or creation, respectively, of these layers, so that layer thickness must not exceed a maximum thickness. This problem occurs in metals such as tungsten, platinum or molybdenum. For a 900 MHz thin film resonator (operating frequency=900 MHz) the optimum thickness for tungsten is around dw=1.4 μm, for platinum around dPt=0.85 μm and for molybdenum around dMo=1.6 μm. Metal layers with such thicknesses can be realized technologically only with difficulty.
Another problem is the parasitic capacitances in the device towards the substrate. For this reason, a maximization of layer thickness is desirable for dielectric layers in the Bragg reflector, such as silicon dioxide, for electrical reasons (minimization of the parasitic capacitances towards the substrate). However, dielectric layers with corresponding thicknesses contradict the above condition (1), as in this case layer thickness exceeds the optimum layer thickness.
Another problem is the different temperature coefficients of the layers 181 to 187. The temperature coefficients of the layers used have an influence on the temperature behavior of the thin film resonator. In the case that the materials for the layers having a high acoustic impedance and for the layers having a low acoustic impedance have temperature coefficients different in sign, a layer thickness combination with minimal temperature coefficients of the thin film resonators can generally be found, however the layers then have thicknesses which do not correspond to the optimum layer thickness, so that such a layer thickness combination generally contradicts the above condition (1).
In the prior art realizations of thin film resonators, as are shown in FIG. 1, are known, in which the acoustic reflector is constructed by means of layers complying with the above condition 1 with regard to layer thickness. However, this results in an undesirable limitation of the choice of the useable materials and the attainable frequency ranges. If, for example, aluminum nitride (AlN) is used as material with high acoustic impedance and SiO2 as material with low acoustic impedance, as is described by R. S. Naik, et al. in IEEE Trans. Ultrasonics, Ferroelectrics, and Freq. Control, 47(1), 2000, pp. 292-296, the above-mentioned problem associated with the parasitic capacitances can be avoided. As the two materials mentioned are dielectrics, this problem does not occur. However, with this material combination the difference between the acoustic impedances is comparatively small, which adversely affects the quality of the Bragg reflector, and is considerably lower with the same number of layers than in Bragg reflectors using one of the above-mentioned metal layers as material with high acoustic impedance.